For some years this laboratory has tested hypotheses which directly or indirectly related to the question of the etiology of trisomy in human subjects. These experiments have been endocrinologic, biochemical and cytogenetic. The present application deals directly with the diplotene stage of mouse and human meiosis with special attention to chiasmata number and location. The development of a capability for exact identification of each bivalent at diplotene makes it possible for the first time to test in mammals the basic genetical concepts of determination of chiasma number and location, sexual dimorphism, terminalization, and interchromosomal effects of structural or numerical abnormalities on chiasmata.